So often when I read about signal transduction in my textbooks they talk about cAMP that acts as second messenger. Only in a very few instances have I read about other second messenger, such as DAG, IP3 and Ca2+.

But how can so many different signals use the same second messenger? It would be more logical for me if every extracellular signal that uses signal transduction to the cell would use an unique second messenger, so that the cell can easily differentiate between different signals.